Retinal degeneration, such as age-related macular degeneration (AMD) and retinitis pigmentosa, currently affects a large portion of the worldwide population, a proportion that is increasing with the ageing of the population.
One avenue explored for treating these pathologies is to carry out an artificial stimulation of the ganglionic neurons with the aim of transferring through the optic nerve a signal that can be used by the visual system.
Such a neuronal stimulation or activation may be artificially induced by pharmacological means or by direct electrical excitation by applying a potential difference on the neuron or the set of neurons to excite.
In the case of a stimulation by pharmacological means, the response and the recovery time may be quite slow, on account of the diffusion time of the pharmacological agent up to the zone of interest. Conversely, electrical stimulation is very rapid, but necessitates an electrical contact with the neuron or the zone to excite. The electrical contact may be made by physical contact of a conductor with the actual neuron or the zone to excite, or by placing a conductor in the immediate vicinity of the neuron or the zone to excite, given the conductivity of the tissues that surround the neuron or the zone to excite.
Artificial electrical retina devices are thus known which comprise systems of implanted microelectrodes in contact with the region to excite. Electrical pulses are delivered by means of an external generator on a number of microelectrodes typically comprised between several tens and several hundreds.
Nevertheless, this technique has certain limitations. A first limitation is due to diffusion of the electrical current in the tissues around the targeted region. In particular, neurons not targeted by the stimulation may be activated via their axon passing in the stimulated region. The spatial selectivity of the excitation and the efficiency of the excitation on the focused zone are thus greatly limited.
A second limitation is linked to the necessity of positioning the implant as near as possible to the neurons to excite, which involves complex and risky implantation surgery. Indeed, whatever the type of prosthesis employed, their positioning has to be done in direct contact with the retina, which imposes potentially impairing sub-or epi-retinal surgery. This mode of implantation in direct contact with the retina also limits the dimensions of the implant to lateral dimensions of the order of 3 mm sides if the implant has a square surface, or 3 mm diameter if the implant has a round surface. At such dimensions, the deformation due to the curvature of the eye is very small, but the implant can thus only activate a very limited portion of the total surface of the retina. Finally, the reaction of the tissues to the presence of the implant may lead in the medium and long term to the development, by an inflammatory reaction and cicatrisation, of granulation tissues, which, in covering the microelectrodes, considerably decrease their efficiency by insulating the microelectrode-tissue interface. This problem is common to implants based on electrical stimulation, such as deep cerebral stimulation implants, or cortical implants within the framework of brain-machine interfaces for example.
It is also known to induce neuronal activation using a magnetic field pulse. Indeed, a rapid variation of magnetic field, created for example by a current pulse in a coil, generates an electric field according to Faraday's law:
      ∇          ×      E        =      -                            ∂          B                          ∂          t                    .      
The interest of the use of the magnetic field pulse on one or more neurons, in terms of clinical application, is linked to the possibility of stimulation of neurons at a distance. Thus, depending on the geometry of the winding, it is possible to create an activation zone at a distance from the exciter coil, and to activate or to inhibit neurons without physical contact therewith, or even with the tissues surrounding them.
At a macroscopic level, this principle is used to stimulate populations of neurons at the scale of the cortical zones of the brain; it is designated Transcranial Magnetic Stimulation. This technique uses large coils with a lateral dimension of 10 to 30 centimeters, in which a current pulse is applied, leading to a rapid variation of the magnetic field and thus the creation of an electric field on a zone of the brain selected by the emplacement and the geometry of the coils. The coils used in transcranial magnetic stimulation are typically planar multi-turn coils, with generally between 10 and 30 turns, of circular, square or rectangular shape. The dimensions of such a coil are typically of the order of 5 to 15 cm side or diameter. Two adjacent coils are generally used in transcranial magnetic stimulation techniques. It has been shown that this technique makes is possible either to activate or to inhibit a neuronal population, as a function of the polarity of the field E with respect to the population. This macroscopic technique is used within the framework of research in cognitive sciences on healthy or pathological brains, as well as in the treatment of psychiatric disorders, for example on depressions resistant to medical treatments.
The document “Functional Magnetic Stimulation for Implantable Epiretinal Prosthesis”, by E. Basham, M. Sivaprakasam and W. Liu (2005) suggests, in order to restore the vision of patients suffering from retinitis pigmentosa or age-related macular degeneration, using magnetic stimulation while pointing to the advantages in terms of biocompatibility and bioresistance compared to electrical stimulation. However, the problem of the complexity of the implantation surgery of such implants remains.